Use -rac_description to prevent unwieldly backtraces
Merge pull request #683 from ReactiveCocoa/sprinkle-rac_description-master

Use -rac_description on master
Refactor -invalidateGlobalRefIfNoNewSubscribersShowUp

Specifically, avoid block allocations (by avoiding variable capture), and only check RACActiveSignals once per run loop iteration.
Don't allocate RACSignal->_subscribers until needed
Avoid -tupleByAddingObject: in zip/combine

This reduces the number of allocations performed as the number of streams grows.
Factor out +zip: and +combineLatest: logic into 1 method
Manage active signals in a lock-free way
Remove temporary arrays and modify RACActiveSignals directly
Document thread requirements for RACActiveSignals
Reinstate comment about signal lifetime
Merge pull request #703 from ReactiveCocoa/lock-free-active-signals

Lock free management of active signals
Merge pull request #699 from ReactiveCocoa/optimizations

Reduce memory consumption, optimize zipping/combining
Disable warnings about repeated __weak usage in tests
Reproducible failing test for #711
Refactor -flatten: to fix crash and use compound disposal
++xcconfigs (via libextobjc)
Remove redundant recursive synchronization in completeSignal()
Revert "Disable warnings about repeated __weak usage in tests"

This reverts commit e7e0abd638a6a9f247a1badb622f0896783394c0.
Merge pull request #713 from ReactiveCocoa/flatten-crash

Refactor -flatten: to fix crash and use RACCompoundDisposable
Add minimal failing test for #714